1. Field of the Invention
The present invention relates to an optical disk apparatus. In particular, the present invention relates to an optical disk apparatus of the type which includes a lens system for forming a laser spot on a disk, wherein the lens system includes a first lens mounted on an actuator supported by a carriage, and a second lens mounted on a slider held in facing relation to the disk. In this specification, the xe2x80x9coptical disk apparatusxe2x80x9d refers not only to a device designed exclusively for reading out data from an optical disk but also to a magneto-optical disk apparatus capable of repeatedly writing data to a magneto-optical disk by magnetic field modulation, light pulse modulation, etc.
2. Description of the Related Art
In general, an optical disk apparatus is arranged to store more data than many other data-storing devices. Thus, various kinds of optical disk apparatus have been developed and widely used as an external storage unit for e.g. computers.
The current data storing capacity of an optical disk apparatus, however, is not large enough to support the full-scale xe2x80x9cMultimedia Eraxe2x80x9d to come. Hence, it is necessary to increase the data-recording density of the optical disk apparatus. For this purpose, the following two techniques may be employed. The first one is to use a semiconductor laser of a shorter wavelength (i.e., a blue semiconductor laser) for a light source, and the second one is to increase the numerical aperture (NA) of an objective used for the optical head.
Blue semiconductor lasers, however, are rather expensive, and their output efficiency is not high enough. Further, they may fail to perform the required functions when the temperature varies. Thus, to use a blue semiconductor laser is not a good idea.
The second option (increasing the NA of the objective) is also disadvantageous in the following points. To increase the NA of an objective, it is necessary to reduce the radius of curvature of the lens surfaces (incidence surface and exit surface). However, to provide the lens surfaces with a small radius of curvature will require a difficult procedure. Further, even if the incidence surface and the exit surface are accurately made and have the intended radius of curvature, they may be tilted or offset relative to each other. Still further, even if the objective lens itself is correctly made (i,e., with the correct radius of curvature and without any erroneous positioning of the two surfaces), the installment of the objective to a supporting member may fail to be performed accurately, thereby rendering the objective slant or offset relative to the supporting member. It is also probable that the optical disk being rotated in operation may be tilted with respect to the objective.
The above-described inappropriateness may give rise to coma (proportional to the NA to the third power). Further, when the optical disk does not have a uniform thickness, spherical aberration (proportional to the NA to the fourth power) will result. The occurrence of coma and spherical aberration may hinder the data-recording and data-reading operations. Since coma and spherical aberration are proportional to the NA of the objective, increasing the NA of an objective may better be avoided.
In this connection, attention should be drawn to e.g. Japanese Patent Application No. 10(1998)-185283 which discloses an optical system consisting of two objectives. It should be noted that this application is laid open on Jan. 21, 2000.
According to the teaching of the above Japanese application, the overall NA of the optical system is increased. In this way, the coma and spherical aberration caused by the increased NA of a single objective are advantageously reduced or even eliminated.
Referring now to FIG. 13 of the accompanying drawings, the principal portions of the optical disk apparatus disclosed in the above Japanese Patent Application (10-185283) will be described below.
Specifically, the optical disk apparatus includes a carriage 10xe2x80x2 which is caused to move radially of an optical disk Dxe2x80x2. The carriage 10xe2x80x2 carries an actuator 20xe2x80x2 which in turn holds a first lens 31xe2x80x2. The actuator 20xe2x80x2 is a two-dimensional actuator which is movable perpendicularly to the disk Dxe2x80x2 for focus control and radially of the disk Dxe2x80x2 for tracking control. A second lens 32xe2x80x2 is arranged above the first lens 31xe2x80x2, supported by an elongated suspension member 40xe2x80x2 fixed to the upper surface of the carriage 10xe2x80x2. The second lens 32xe2x80x2 is held by a slider 41xe2x80x2 mounted on the upper end of the suspension member 40xe2x80x2.
The first lens 31xe2x80x2 and the second lens 32xe2x80x2 are arranged vertically so that their optical axes coincide with each other. With such an arrangement, even if the NA of each lens is rather small (meaning that the lens is easy to process), the overall NA of the optical system as a whole can be rendered sufficiently large. Further, in the optical head, the slider 41xe2x80x2 is pivotably attached to the upper end of the suspension member 40xe2x80x2. Thus, when the slider 41xe2x80x2 is brought close to the disk Dxe2x80x2, the slider can automatically adjust its posture to be held in close facing relation to the surface of the rotating disk Dxe2x80x2. In this manner, the coma due to the tilting of the disk Dxe2x80x2 is advantageously prevented from occurring. In addition, the spherical aberration due to an uneven thickness of the disk Dxe2x80x2 is avoided by adjusting the distance between the first and the second lenses 31xe2x80x2, 32xe2x80x2 by moving the actuator 20xe2x80x2 perpendicularly to the disk Dxe2x80x2.
While the above optical head has various advantages, it may suffer the following problem. Ideally, the optical axis of the first lens 31xe2x80x2 coincides with the axis of the second lens 32xe2x80x2, as shown in FIG. 14. However, since the slider 41xe2x80x2 is supported by the elongated suspension member 40xe2x80x2, the optical axis L1xe2x80x2 of the first lens 31xe2x80x2 may be horizontally displaced from the optical axis L2xe2x80x2 of the second lens 32xe2x80x2 (see FIG. 15). This may be caused in part by an error in fixing the slider 41xe2x80x2 to the suspension member 40xe2x80x2 and/or an error in fixing the suspension member 40xe2x80x2 to the carriage 10xe2x80x2. Another cause of the deviation of the axes L1xe2x80x2, L2xe2x80x2 may be thermal expansion of the suspension member 40xe2x80x2 or slider 41xe2x80x2. In this case, the deviation of the optical axes L1xe2x80x2, L2xe2x80x2 may begin to occur after the optical disk apparatus is turned on. Thereafter, the deviation may expand as the suspension member 40xe2x80x2 and the slider 41xe2x80x2 are being heated up.
FIG. 16 is a graph showing the relation between the aberration and the discrepancy between the two optical axes L1xe2x80x2, L2xe2x80x2. This graph clearly shows that the aberration increases as the two optical axes are spaced further away from each other. As stated above, the actuator 20xe2x80x2 carrying the first lens 31xe2x80x2 is movable to perform the focus control and the tracking control. Considering this function, the discrepancy between the two axes L1xe2x80x2, L2xe2x80x2 may need to be smaller than 40 xcexcm for example, so that sufficient data-reading and data-writing margins are ensured. However, the extent of the thermal expansion of the suspension member 40xe2x80x2 and/or slider 41xe2x80x2 is often unpredictable. Thus, it is difficult or even impossible to assembly the optical head in a manner such that the discrepancy between the two optical axes L1xe2x80x2, L2xe2x80x2 is to be below 40 xcexcm.
It is, therefore, an object of the present invention to provide an optical disk apparatus with an optical head which includes an objective lens system consisting of a plurality of lenses, wherein the offset between the optical axes of the respective lenses is adjusted.
Another object of the present invention is to provide a method of adjusting lenses used in an optical disk apparatus.
According to a first aspect of the present invention, there is provided an optical disk apparatus comprising: a carriage movable relative to a data-storing disk member; an actuator mounted on the carriage and movable at least for performing focus control; a first lens supported by the actuator and having a first optical axis; a suspension member provided with a first end and a second end, the suspension member being supported by the carriage via the first end; a slider attached to the second end of the suspension member and brought into facing relation to the disk member; a second lens supported by the slider and having a second optical axis, the second lens being associated with the first lens to make a light spot on the disk member; and optical axis adjusting means provided separately from the actuator and arranged to move the first and the second optical axes relative to each other in parallel to the disk member.
With such an arrangement, a discrepancy between the first and the second optical axes is advantageously eliminated or reduced by operating the optical axis adjusting means.
According to a preferred embodiment of the present invention, the optical axis adjusting means may include a first shifting mechanism and a second shifting mechanism. The first shifting mechanism may move the first and the second optical axes relative to each other in a predetermined direction parallel to the disk member, while the second shifting mechanism may move the two optical axes relative to each other in another direction which is perpendicular to said predetermined direction and parallel to the disk member.
Preferably, the first shifting mechanism may cause the carriage and the disk member to be moved toward and away from each other.
According to a preferred embodiment, the first end and the second end of the suspension member may be spaced from each other tangentially to a track of the disk member.
In the above instance, the second shifting mechanism may include an additional actuator mounted on the carriage for moving the suspension member radially of the disk member.
Preferably, the actuator may be a two-dimensional actuator movable for positional adjustment of the first and the second optical axes.
Preferably, the additional actuator may comprise a piezoelectric element.
According to another preferred embodiment, the first end and the second end of the suspension member may be spaced from each other radially of the disk member.
In the above case, the second shifting mechanism may include an additional actuator mounted on the carriage for moving the suspension member tangentially to a track of the disk member.
According to a preferred embodiment of the present invention, the first shifting mechanism may include a first additional actuator for moving the suspension member tangentially to a track of the disk member, while the second shifting mechanism may include a second additional actuator for moving the suspension member radially of the disk member.
Preferably, the first shifting mechanism may include at least one elongated guiding member movably supporting the carriage. The guiding member may have an end portion which is pivotably connected to a suitable supporting member of the optical disk apparatus.
Preferably, the first shifting mechanism may include a cam held in engagement with the guiding member at a predetermined portion thereof spaced from the pivotably connected end.
With such an arrangement, the guiding member can be moved upward and downward about the pivot upon actuation of the cam.
In the above case, the carriage may be reciprocated between said predetermined portion and the pivotably connected end.
According to another preferred embodiment of the present invention, the first shifting mechanism may include a cam for causing the disk member to move toward and away from the carriage.
According to another preferred embodiment, the optical disk apparatus may comprise a spindle for rotating the disk member, wherein the cam is held in engagement with the spindle.
In the above case, the optical disk apparatus may further comprise a cylindrical shaft for holding the spindle in an axially slidable manner.
According to a second aspect of the present invention, there is provided a method of adjusting positions of first and second lenses used in an optical disk apparatus, wherein the first lens has a first optical axis, and the second lens has a second optical axis. The method comprises the steps of: shifting the first and the second optical axes relative to each other; writing data to a recording disk; reading out the data for obtaining a readout signal; and determining optimum positions of the first and the second optical axes based on the readout signal.
According to a third aspect of the present invention, there is provided a method of adjusting positions of first and second lenses used in an optical disk apparatus using a storage disk. The first lens has a first optical axis, while the second lens has a second optical axis. The method comprises: a first step of offsetting the first lens from the second lens to maximum in a predetermined direction; a second step of writing trial data to the storage disk; a third step of reading out the trial data from the storage disk; a fourth step of calculating an error rate in reading out the trial data; a fifth step of storing the calculated error rate in a memory; a sixth step of moving the first lens relative to the second lens by a predetermined amount; a step of repeating the second through the sixth steps until the first lens is offset to maximum in another direction opposite to said predetermined direction; and a step of finding a position of the first lens relative to the second lens at which the error rate is minimized.